1. Field of the Invention
The present invention relates to a solid state imaging device, a method for producing the solid state imaging device, and an electronic apparatus including the solid state imaging device.
2. Description of the Related Art
Solid state imaging devices are broadly divided into charge-transfer-type solid state imaging devices represented by CCD (charge coupled device) image sensors and amplification-type solid state imaging devices represented by CMOS (complementary metal oxide semiconductor) image sensors.
CMOS solid state imaging devices are operable at low power supply voltage and hence advantageous in view of power consumption compared with CCD solid state imaging devices. For this reason, CMOS solid state imaging devices are used as solid state imaging devices mounted on mobile apparatuses such as camera-equipped cellular phones and PDAs (personal digital assistants).
A photodiode serving as a photoelectric conversion portion constituting a pixel of a solid state imaging device and using electrons as signal charge is constituted by an n-type semiconductor region and a p-type semiconductor region on the surface of the n-type semiconductor region. As disclosed in Japanese Unexamined Patent Application Publication No. 2005-191262, an existing photodiode of a solid state imaging device is typically constituted by a p-type semiconductor region having a high impurity concentration in the top surface (interface) of a p-type semiconductor well region of a semiconductor substrate and an n-type semiconductor region under the p-type semiconductor region. Such a photodiode accumulates charge generated by photoelectric conversion at pn junction capacitance between the p-type semiconductor region and the n-type semiconductor region, that is, at depletion-layer capacitance. To isolate neighboring pixels from each other, for example, a device isolation region is used that is formed by filling a trench with an insulation film. In this case, a p-type semiconductor region is used to protect the interface between the photodiode and the insulation film.
Japanese Unexamined Patent Application Publication No. 2003-318383 also discloses a photodiode serving as a photoelectric conversion portion in a solid state imaging device. A photodiode of a CCD solid state imaging device in Japanese Unexamined Patent Application Publication No. 2003-318383 is constituted by an n-type semiconductor region and a p-type semiconductor region on the top surface of the n-type semiconductor region. This p-type semiconductor region is constituted by a first p-type semiconductor region and a second p-type semiconductor region. The first p-type semiconductor region is formed on the whole surface of the n-type semiconductor region. The second p-type semiconductor region is formed so as to have a smaller width than that of the n-type semiconductor region and is formed deeper than the first p-type semiconductor region. Such a two-level structure of the p-type semiconductor region results in mild variation in field intensity at the interface between the p-type semiconductor region and a vertical transfer region. Thus, charge can be efficiently transferred to the vertical transfer region.
Japanese Unexamined Patent Application Publication No. 2005-223084 discloses a CMOS solid state imaging device having a configuration in which a vertical transistor is used as a transfer transistor among pixel transistors. This CMOS solid state imaging device has a floating diffusion (FD) portion over a photodiode and a transfer gate electrode is formed over the FD portion with a gate insulation film therebetween so as to extend in the depth direction of a substrate.